The present invention relates to a process for the production of isocyanates in the gas phase in which temperature fluctuations over time and temperature distribution asymmetry are prevented. This is achieved by improved performance of the reaction in tubular reactors by flow-related measures such as homogenization and centering of the educt streams. The formation of polymeric secondary products, which result in baked-on deposits in the reactor and a reduction in reactor service life, is consequently prevented.
EP-A 0 289 840 describes a process for the production of (cyclo)aliphatic diisocyanates by phosgenation of the corresponding, vaporous (cyclo)aliphatic diamines at 200xc2x0 C. to 600xc2x0 C. Phosgene is supplied in stoichiometric excess. The superheated streams of vaporous (cyclo)aliphatic diamine or (cyclo)aliphatic diamine/inert gas mixture on the one hand and of phosgene on the other hand are passed continuously into a cylindrical reaction chamber, where they are mixed together and caused to react. The exothermic phosgenation reaction is performed while maintaining a turbulent flow.
Gaseous educts are frequently reacted in tubular reactors. If the jet mixer principle is applied (Chemie-Ing.-Techn. 44 (1972), p. 1055, FIG. 10), two educt streams, A and B are supplied to the tubular reactor. Educt stream A is supplied via a central nozzle and educt stream B via an annular space between central nozzle and tubular reactor wall. The flow velocity of the educt stream A is high relative to the flow velocity of educt stream B. This causes intensive mixing of and consequently reaction of the reaction partners in the tubular reactor. This method of performing the reaction has become industrially significant in the production of aromatic diisocyanates by phosgenation of aromatic diamines in the gas phase (e.g. EP-A 0 570 799).
However, the known processes exhibit major temperature fluctuations of up to 50xc2x0 C. during performance of the reaction. Moreover, temperature distribution asymmetry of up to 100xc2x0 C. may be measured in the circumferential direction of the cylindrical reaction chamber or of the tubular reactor, e.g. using thermocouples.
A consequence of the temperature fluctuations and temperature distribution asymmetries is the formation of polymeric secondary products, which result in baked-on deposits and blockages in the reactor and thus in a reduction in the service life of the reactors.
The object of the invention is therefore to provide a process for the production of (cyclo)aliphatic and aromatic diisocyanates by phosgenation of corresponding (cyclo)aliphatic and aromatic diamines in the gas phase at high temperatures, in which process the temperature fluctuations in the reaction zone and the temperature distribution asymmetries are prevented to the greatest possible extent.
It has now surprisingly been found that homogenization of the educt stream B supplied via the annular space of the tubular reactor and maximally central supply of the two educt streams A and B into the tubular reactor have a marked positive influence on the stability of the reaction zone and thus on the gas phase reaction overall. As a consequence of performing the reaction in a more stable-manner, the observed temperature fluctuations diminish markedly, and the temperature distribution asymmetry disappears virtually completely. Thus, the disadvantages of the prior art process may be markedly reduced, by subjecting the educt streams to the measures according to the inventions described below in more detail.